Numerous problems are encountered when thermoforming plastic articles, such as plastic cups. However, some plastic materials such as polypropylene, tend to droop or sag after they are heated and while they are being delivered into a thermoforming machine for molding between co-acting male and female dies.
FIG. 1 illustrates one present system for supporting plastic sheet material that tends to droop during a thermoforming operation. More particularly, a thermoforming machine 10 is provided with a presently understood mold assembly including an upper female die platen 12, a lower male die platen 14, a clamping grid 16 supported above a thermoformable plastic sheet 34, conveyor rails 24,26, sag rails 28-30, and a heating oven 32. In order to accommodate types of plastic sheet material 34 that tend to droop via gravity when heated, conveyor rails 24, 26 are splayed outwardly in a downstream travel direction. Splaying of the rails can over-stress the rails, leading to excessive wear and eventual break down. Additionally, sag rails 28-30 are provided through oven 32 and immediately upstream and adjacent to clamping grid 16 of thermoforming machine 10 to support sheet 34 from sagging and to reduce or (hopefully) eliminate contact of sheet 34 with the top of clamping grid 22. Such contact can damage or ruin a surface finish on sheet 34, which can also damage surface finish on products molded from thermoforming machine 10. For example, contact of heated sheet 34 with a top of clamping grid 16 will cool sheet 34 at the contact site, which tends to cool the sheet at the contact point, resulting in non-uniform heat within the sheet. This can negatively affect formability. Furthermore, it can produce wrinkles in the sheet 34.
In operation, female dies 18 are provided on a bottom surface of platen 12 within a female die plate that is affixed to platen 12. Individual male dies 20 are also provided atop a male die plate mounted atop platen 14. Bores 22 are provided through clamping grid 22. In operation, clamping grid is held stationary, just below sheet 34 which is delivered directly above. Platen 12 is driven downwardly to clamp sheet 34 against the top of grid 22, while sheet 34 is held stationary. Subsequently, platen 14 is driven upwardly to drive male dies 20 through bores 22, drawing sheet 34 into female dies 18.
Improvements are needed to eliminate the need for sag rails and splaying of conveyor rails 24,26 in order to reduce or eliminate sag of sheet 34 during a thermoforming operation.